Related Stories

Romantic cost Walking out with your sweetheart could be a waste of energy if you're a man, say researchers.

They say this is because when men walk with a woman they're romantically involved in, they walk slower than their optimum speed, and are thus less energy efficient.

"Men slowed down about 7 per cent and this was significantly slower than when they walked alone," says lead author, biological anthropologist Dr Cara Wall-Scheffler of the Seattle Pacific University.

The optimum walking speed is the one that uses the least amount of energy to cover a particular distance, says Wall-Scheffler.

She says energy is lost with every step - when your foot hits the ground, when you have to lift your body and move it forward, and when you have to swing your leg.

"If you can reduce the number of steps you are taking while you're walking then you can reduce the amount of energy you take to go a given distance," says Wall-Scheffler.

In previous research, she found that men tend to have a higher optimum speed than women because they tend to be taller than women and take longer strides.

But what happens when people with different optimal walking speeds walk together? Who slows down and who speeds up?

To investigate this question, Wall-Scheffler studied what happened when different pairs of people walked together. Her study is reported today in the journal PLOS ONE.

Walking study

Wall-Scheffler and colleague Janelle Wagnild observed what happened when 11 couples, and their male and female friends, walked around a track in different combinations.

All the men were about the same height as each other, and all the women were about the same height as each other, but shorter than the men.

Walking alone, the men had an average optimum speed of 1.53 metres per second whereas the women walked at 1.44 metres per second.

Wall-Scheffler found that when a pair of female friends or a pair of male friends walked together, they walked at their optimum speed.

When a man walked with a woman friend, he tended to walk close to his optimum speed and the woman sped up slightly. But a man changed his walking speed when he was with a woman he was romantically involved in.

"The men only slowed down to walk as slowly as women when they were in love with the woman," says Wall-Scheffler.

"When they were holding hands they slowed down even more."

This slowing down costs men energetically, says Wall-Scheffler.

"If you walk away from your energetic optimum then you're burning a lot of calories and for men in particular they burn quite a [few] calories when they are walking more slowly than their optimum speed," she says.

Evolutionary consequences

Wall-Scheffler says the findings have evolutionary significance because as hunter-gatherers, walking large distances, slowing down to walk with his partner could cost a man up to 10 per cent of his daily energy expenditure.

"Hunter-gatherers usually walk in single sex groups," she says. "This fits into our data."

This ensures neither men nor women develop an "energetic burden" by walking slower or faster than their optimum speed.

The findings may help interpret fossil footprints, says Wall-Scheffler.

"If there are just a few individuals ... and the individuals have reasonable size difference ... then the individuals were likely a mated pair," she says.

The study also suggests by walking slower with their partner men can also help protect female fertility, adds Wall-Scheffler.

The more energy you use for locomotion, the less energy you have for other things, including reproduction.

So by a couple slowing down women can save much-needed energy for reproduction.